Not applicable.
Not applicable.
The present invention relates generally to optical fibers with a lens formed on an end of the optical fiber, and more specifically to a lensed optical fiber with improved optical coupling.
Optical fibers are used in a variety of optical telecommunications applications. They are commonly used for the transmission of information, such as data or voice information, over relatively long distances with high rates of transmission, commonly referred to as xe2x80x9cbandwidthxe2x80x9d, and immunity from electronic noise. Optical fibers are also used in a wide variety of optical components, such as optical switches, tunable optical sources, optical amplifiers, and pump sources for optical amplifiers. An optical fiber xe2x80x9cpigtailxe2x80x9d, or several optical fiber pigtails, is often provided so that an optical component can be connected to a transmission network. In some instances, the end of the optical fiber pigtail is fusion spliced (i.e. melted) to a fiber end of the network or another optical fiber pigtail. Other methods for joining fibers, such as by aligning them in close proximity in a capillary tube in a ferrule, are also known.
When an optical fiber pigtail is provided from a packaged optical component, the end of the optical fiber pigtail within the package typically is optically coupled to some other element or elements within the package. This optical coupling has been approached in a variety of ways. In one optical switching technique, the end of one fiber is physically moved across other fiber ends to be aligned with the fiber end desired by the switching function. Optical coupling occurs between fibers without any particular collimation or focusing of the light. The light on one fiber, which forms an optical waveguide having a core and a cladding, continues down the fiber aligned to the first.
Another approach is to expand the light from a fiber end into a light beam with a lens or lenses, and to manipulate that light beam in free space. For example, the light beam could be directed at a prism, optical filter, or mirror, and gathered by another lens or lenses to be focused onto the end of another optical fiber. This technique is commonly referred to as collimation. The collimation lenses are often nominally quarter-wave gradient-refractive-index (xe2x80x9cGRINxe2x80x9d) lenses that are relatively large and often require additional hardware, such as ferrules, to optically couple the optical fiber to the lens. Unfortunately, the size and characteristics of these lenses make them unsuitable for some applications.
For example, optical fiber amplifiers are typically coupled to a pump module. One type of pump module contains a semiconductor waveguide laser diode chip that emits light for pumping the optical fiber amplifier. The optical fiber amplifier is typically connected to the pump module with an optical fiber pigtail from the module. The other end of the optical fiber pigtail is placed in close proximity to the output of the laser diode chip to couple as much of the light from the laser diode chip as possible. It is generally desirable to put the end of the fiber in close proximity to the laser diode chip because light coming out of the laser diode chip disperses, reducing its intensity. The size of a conventional cylindrical GRIN lens makes it undesirable for this sort of application.
An alternative approach has been to shape the end of the fiber into a xe2x80x9cmicrolensxe2x80x9d. One lens shape that has been used is a chisel lens. A chisel lens generally has two surfaces ground or lapped onto the end of the optical fiber so that a generally rounded line intersecting the optical axis of the fiber is formed. The resultant end of the fiber can operate as a lens to gather light from a source, such as a pump laser diode chip. Chisel lenses provide superior coupling compared to a cleaved fiber end, for example, but it is generally desirable to improve optical coupling, and further refinements to chisel lenses have been made. For example, chisel lenses have been formed at an angle to the optical axis of the fiber, as shown and discussed in U.S. Pat. No. 5,940,557 by Andrew Harker. Similarly, anti-reflective coatings have been added to fiber ends to reduce reflective loss and to reduce back-reflections into the pump laser diode or other element.
However, it is desirable to provide a lensed optical fiber with a microlens that enhances optical coupling and reduces reflection.
The present invention is an optical fiber with a microlens formed on an end of the optical fiber. In one embodiment, a double-chisel lens is formed on the fiber end. This lens gathers light in both the slow and fast directions when coupling to a typical pump laser diode. In another embodiment, a pointed microlens is ground or lapped with centers offset from the central axis of the fiber. In an alternative embodiment, the lens is pointed by flat, rather than radiused, surfaces. In another embodiment, a fiber end is lensed in the fast and slow directions, in other words, doubly lensed, without forming a double chisel structure.
In a particular embodiment, a lensed optical fiber with a pointed microlens has a first surface with a first center of curvature offset from a center axis of the lensed optical fiber and a second surface having a second center of curvature also offset from the center axis of the fiber. A tangent line from the first surface at the point forms an angle of between about 176-156 degrees with a tangent line from the second surface at the point.
In another embodiment, a lensed optical fiber has a double-chisel lens. Each side of the lens has two chisel faces at a slight angle to each other and at least one is angled with respect to the center axis of the fiber, forming a ridge essentially from the tip of the lens back to the outer surface of the fiber. In one embodiment each chisel face is formed at a slight angle to the center axis and the tip is formed in the core portion of the fiber. Each of these chisel faces forms a ridge with corresponding opposing faces on the other side of the lens, such as with a conventional chisel lens, and this second ridge is essentially orthogonal to the ridges formed between chisel faces on each side of the lens.
The double chisel structure provides a cut-away portion for aligning the fiber close to a light source. In one embodiment, a section along one ridge through the tip of the lens is pointed, and along the other ridge through the tip is radiused. This structure reduces back reflections while providing lensing effect along both axes. Generally, the section through the intersection of the chisel faces and the tip is pointed and the orthogonal section is radiused. The radius of curvature is about 12-22 microns in one embodiment, and about 5-11 microns in another embodiment. In an alternative embodiment, the tip is not pointed, but is radiused with a radius of curvature of about 12-22 microns in one direction and about 5-11 microns in the orthogonal direction. In a particular embodiment, the smaller radius of curvature is in the plane defined by the ridges formed by the intersections of the chisel faces on each side.
In a further embodiment, the ridges in the horizontal direction in a double-chisel lens each form an angle of between about 2-12 degrees with a plane orthogonal to the center axis of the optical fiber. However, one angle is at least 1-3 degrees different than the other. This accommodates misalignment in the subassembly of the light source and laser mount by allowing the assembler to try both orientations (generally aligning these ridges in a horizontal or nearly horizontal orientation with respect to the surface of the substrate).